Photographic processes involving the use of silver halides have been widely used heretofore because of their superior photographic properties, such as sensitivity, tone adjustment etc., in contrast to other photographic processes such as electrophotography or diazo photography.
In recent years, techniques have been developed for obtaining images easily and rapidly using silver halide photographic materials by processing them in the dry state, such as by heating, instead of conventional wet treatments such as using developer solutions.
Thermodevelopable materials are known in the art, and thermodevelopable materials and processes therefore are described, for example, in Fundamentals of Photographic Engineering (a Japanese-language publication published by Corona Co. in 1979), pages 553-555; Image Information (published April 1978), page 40; Nebletts Handbook of Photography and Reprography, 7th Ed., 1977 published by Van Nostrand Reinhold Company, pages 32-33, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, June 1978, pages 9-15 (RD No. 17029).
Nowadays, in the field of image information, black-and-white images have been largely replaced by color images because of the large quantities of information and the ease of expression, but in many fields, for example in medical therapy, black-and-white images are still preferred.
Commercially available photographic materials based on a system in which the unused silver halide is left within the photographic materials after processing are known. Thus, upon exposure to strong light or long-term storage, the remaining silver halides or silver behenate gradually reacts to cause coloration of the white background. Furthermore, according to this system, the silver cannot be recovered, unlike processes using color coupling, and this gives rise to a problem in regard to resources.
Research Disclosure, September 1978, pages 49-51 (RD No. 17326) discloses a process in which a black color image is obtained by dry processing. This process is also of the unfixed type with the inclusion of silver or silver halide in the color image, and has the same defect as stated above. No method has so far been known which can give a black-and-white image free from silver or a silver salt by dry processing.